1. Field of the Invention
The present invention relates to a capacitor charging circuit for charging a capacitor via a flyback transformer, and a strobe apparatus that lights a light emitting tube via the capacitor charging circuit.
2. Description of the Related Art
Conventional strobe apparatuses of this type are disclosed in Japanese Patent Application Laid-open No. 2002-152987, Japanese Patent Application Laid-open No. 2002-359095 and the like. FIG. 3 shows an example of a conventional strobe apparatus of this type. The strobe apparatus 101 includes a capacitor charging circuit 102 and a light emitting tube 3. The capacitor charging circuit 102 includes a flyback transformer 110 having a primary coil 111, one end of which is connected to the input power source VCC, and a secondary coil 112 that outputs a secondary coil current from one end, a switching element 114 that is an N-type MOS transistor connected to the other end of the primary coil 111, for turning on and off the primary coil current Ia that flows in the primary coil 111, a resistor 115 for measuring the primary coil current Ia, one end of which is connected to the source of the switching element 114 and the other end of which is connected to ground, a capacitor 117 that is charged via a rectifier diode 116 by the secondary coil current Ib that is produced through the on-off action of the switching element 114 and flows through the secondary coil 112, a serial connection circuit including a resistor 119 and a Zener diode 118 arranged in parallel to the capacitor 117 for measuring the charge voltage of the capacitor 117, a diode 120 for measuring the secondary coil current Ib, the cathode of which is connected to the other end of the secondary coil 112 and the anode of which is connected to ground, a resistor 121 with a high resistance value that biases the cathode of the diode 120 to ground potential, and a charge control circuit 122 that inputs the voltage at one end of the resistor 115 to input terminal A, the voltage at the cathode of the diode 120 to input terminal B, and the voltage at the point of connection between the Zener diode 118 and the resistor 119 to input terminal C, and outputs from output terminal D the on-off signal of the switching element 114 generated based on these voltages. This charge control circuit 122 starts when a command signal to begin the on-off action of the switching element 114 is sent from the strobe control circuit (not shown) that controls the strobe apparatus 101 as a whole. The light emitting tube 3 is lit by discharging the charge that has accumulated in the capacitor 117.
In this capacitor charging circuit 102, when the switching element 114 is on, the primary coil current Ia increases linearly, and energy is stored in the flyback transformer 110. This primary coil current Ia is detected by the voltage at one end of the resistor 115 (the voltage of the input terminal A). When the current reaches a predetermined current value, the switching element 114 turns off by the charge control circuit 122. When the switching element 114 is off, the secondary coil current Ib flows in the secondary coil 112 decreasing linearly. The capacitor 117 is charged via the rectifier diode 116, and the energy stored in the flyback transformer 110 is decreased. The secondary coil current Ib is detected by the voltage at the cathode of the diode 120 (the voltage of the input terminal B). When this current reaches a value close to zero, the switching element 114 is turned on by the charge control circuit 122. Then the primary coil current Ia increases linearly again and energy is stored in the flyback transformer 110.
By repeating the on-off action of the switching element 114 in this way, the charge voltage of the capacitor 117 gradually increases but the upper limit of this charge voltage is fixed by the breakdown voltage of the Zener diode 118. Until the charge voltage of the capacitor 117 reaches the breakdown voltage, no current flows in either the Zener diode 118 or the resistor 119. When the charge voltage of the capacitor 117 does reach the breakdown voltage, a current flows through the Zener diode 118 and the charge voltage of the capacitor 117 is maintained at a constant level. At this time, a current also flows through the resistor 119. The voltage at the point of connection between the Zener diode 118 and the resistor 119 (i.e., the voltage of the input terminal C of the charge control circuit 122) increases, whereupon the charge control circuit 122 determines that the charge voltage of the capacitor 117 is sufficient and stops the on-off action of the switching element 114. In this way, the capacitor charging circuit 102 is controlled once the charge voltage of the capacitor 117 reaches a predetermined voltage, thereby preventing any wasted current consumption.
However, the Zener diode used in the capacitor charging circuit 102 described above is not suitable for being integrated within a semiconductor integrated circuit since high voltages are applied to it at both ends. For this reason, a stand-alone standard component of the Zener diode is commonly used. The large size of such a stand-alone standard component means that it takes up a large area on the printed circuit board and also results in higher costs.